1. Field of the Invention
The present invention relates to a thermal recording material, and more particularly to a thermal recording material having an excellent resistance to alcoholic flexographic ink.
2. Discussion of the Related Art
With diversification of information and expansion of needs, various types of recording materials have been researched and developed and are in practical use in the field of information recording. In particular, thermal recording materials are widely used in an information processing field (output of a desk-top calculator, computer or the like), a medical measurement recorder field, a low or high speed facsimile field, an automatic ticket machine field (railway tickets, admission tickets or the like), a thermal copying machine field and a label field for a point of sale (POS) system because of having advantages such that (1) images can be recorded only by applying heat; (2) the mechanism of recording apparatus is simple so that the apparatus can be miniaturized, and a recording material is easy to handle and inexpensive.
A need exists for a thermal recording material which quickly forms a color image having a high image density, wherein the image and the background have high durability. In recent years, thermal recording materials have been used in a large quantity in fields such as label printing, in which recorded images are required to have high reliability, and therefore there is a demand for a thermal recording material having a good preservation stability against plasticizers or oils and fats contained in an organic polymeric material used for a wrapping material.
In order to overcome the drawbacks, a protective layer has been conventionally located overlying a thermal recording layer. In particular, it is proposed that polyvinyl alcohol and modified polyvinyl alcohol are used as a resin in a protective layer, and a combination of these polyvinyl alcohols and a waterproof agent is used as a protective layer.
In addition, as a recent trend, the demand for the thermal recording material is shifting from POS labels for use in a food industry to labels for use in distribution and ticket industries. The thermal recording material is required to have good durability in a printing process and good qualities such as high speed printability (high heat sensitivity) rather than resistance to a plasticizer contained in a polyvinyl chloride wrapping film.
In order to meet the above quality requirements, not only investigation of dyes and developers and utilization of an intermediate layer, but also formation of a thin protective layer (about from 1 μm to 2 μm) are necessary. In particular, it is necessary for the thin protective layer to have excellent resistance to alcoholic flexographic ink.
In attempting to impart good waterproofing property to a protective layer, published unexamined Japanese Patent Applications Nos. S61-95978 and H11-302331 have disclosed that zirconium oxychloride, zirconium sulphate, zirconium nitrate, carbonic acid zirconium, stearic acid zirconium, octyl acid zirconium and silicic acid zirconium compounds are used in a protective layer. However, almost all the above-mentioned compounds cannot impart good resistance to water. In addition, compounds having resistance to water such as silanol modified PVA and carbonic acid zirconium ammonium are also proposed, but these compounds do not have good resistance to alcoholic flexographic ink. In addition, published unexamined Japanese Patent Application No. 2001-138637 discloses an organic titanium compound having good resistance to water, but the compound does not have good resistance to alcoholic flexographic ink. In addition, the thermal recording material causes a problem in that the background of images yellows under high humidity conditions.